1. Field of the Invention
The present invention relates to a device and a method for driving an active matrix light-emitting display panel which selectively luminescently drive a large number of light-emitting elements which exhibit different emission colors by using, e.g., TFTs (Thin Film Transistors).
2. Description of the Related Art
Along with the popularization of a mobile telephone, a personal digital assistant (PDA), and the like, a demand for a display panel which has a high-definition image display function and can realize a small thickness and a low power consumption increases. As a display panel which satisfies the demand, liquid crystal panels are conventionally applied to a large number of products. On the other hand, in recent years, an organic EL (Electro-Luminescence) element which takes advantage of characteristics of a self-emitting display element is practically used. The display panel draws attention as a next-generation display panel which is replaced with a conventional liquid crystal display panel. This is caused by a background in which an organic compound which can expect preferable light-emitting characteristics is used in a light-emitting function layer of an element to achieve practical high efficiency and practical long life.
The organic EL element, for example, is basically formed such that a transparent electrode consisting of, e.g., ITO, a light-emitting function layer consisting of an organic material, and a metal electrode are sequentially stacked on a transparent substrate such as a glass substrate. The light-emitting function layer may be a single layer consisting of an organic light-emitting layer, a two-layer structure consisting of an organic hole transportation layer and an organic light-emitting layer, a three-layer structure consisting of an organic hole transportation layer, an organic light-emitting layer, and an organic electron transportation layer, or a multi-layer structure obtained by inserting an electron or hole-implanted layer between these appropriate layers.
The organic EL element can be electrically expressed by an equivalent circuit as shown in FIG. 1. More specifically, the organic EL element can be electrically replaced with a configuration constituted by a diode component E serving as a light-emitting element and a parasitic capacitive component Cp coupled in parallel to the diode component E. The organic EL element is considered as a capacitive light-emitting element.
When a light-emitting drive voltage is applied to the organic EL element, first, electric charges corresponding to the electric capacitance of the element flow into the electrode as a displacement current and are accumulated in the electrode. Subsequently, when the voltage exceeds a predetermined voltage (light-emitting threshold voltage=Vth) inherent in the element, a current begins to flow from one electrode (anode side of the diode component E) to the organic layer constituting the light-emitting layer. It can be understood that light emission occurs with an intensity which is in proportion to the current.
FIGS. 2A to 2D show light-emitting static characteristics of such an organic EL element. According to this, the organic EL element, as shown in FIG. 2A, emission occurs with a luminance L which is appropriately proportional to a drive current I. As indicated by a solid line in FIG. 2B, a drive voltage V is equal to or higher than an emission threshold voltage Vth, the current I rapidly flows to emit light.
In other words, when the drive voltage is equal to or lower than the emission threshold voltage Vth, a current rarely flows in the EL element, and the EL element does not emit light. Therefore, the EL element has the following luminance characteristic. That is, as indicated by a solid line in FIG. 2C, in an emittable region in which the drive voltage is larger than the threshold voltage Vth, as the voltage V applied to the EL element increases, an emission luminance L increases.
On the other hand, it is known that the organic EL element has physical properties which change in long-term use to increase a forward voltage Vf. For this reason, In the EL element, as shown in FIG. 2B, a V-I (L) characteristic changes in a direction indicated by an arrow (characteristic indicated by a broken line) depending on actual operating time. Therefore, the luminance characteristic also decreases.
Furthermore, it is known that the luminance characteristic generally changes as indicated by a broken line in FIG. 2C depending on a temperature. More specifically, the EL element has the following characteristics. That is, in an emittable region in which the drive voltage is larger than the emission threshold voltage, as the voltage V applied to the EL element increases, the emission luminance L of the EL element increases. However, the temperature increases, the emission threshold voltage decreases. Therefore, a minimum applied voltage with which the EL element is set in an emittable state decreases as the temperature increases. Even though a predetermined emittable applied voltage is given, the EL element is bright at a high temperature and dark at a low temperature. That is, the luminance is dependent on temperature.
In addition, the EL elements disadvantageously have luminous efficiencies to a drive voltage which change depending on emission colors. As the luminous efficiencies of EL elements which can be practically used and emit R (Red), G (Green), and B (Blue) lights, in an early stage, as generally shown in FIG. 2D, the emission efficiency of G is high, and the emission efficiency of B is the lowest. Each of the EL elements which emit R, G, and B lights has an aging characteristic and a temperature dependence as shown in FIGS. 2B and 2C.
Therefore, when EL elements which emit R, G, and B lights are arranged as sub-pixels to try to perform, e.g., full-color display, a color balance is disrupted due to a change in environment temperature or aging, and display quality cannot be easily held at a predetermined level. In particular, in a device for driving an active matrix display panel having a configuration in which EL elements are driven at a constant voltage by switching operations of TFTs, as indicated by V-I (L) characteristics shown in FIGS. 2A to 2D, an emission luminance largely varies with a variation of the forward voltage Vf of each element to pose a problem of considerable deterioration of display quality.
For this reason, in order to solve the above problem, monitor elements which monitor the forward voltages Vf of the EL elements which emit R, G, and B lights are prepared. A device for driving a light-emitting display panel in which drive voltages applied to the EL elements which emits the color lights are independently controlled based on the forward voltages Vf obtained by the monitor elements is disclosed in Japanese Unexamined Patent Publication No. 2003-162255.
As described above, when the drive voltages applied to sub-pixels which emit R, G, and B lights are independently controlled in accordance with the aging or the like, the drive circuit constituted by TFTs which luminescently drive EL elements constituting the R, G, and B light-emitting elements are inhibited from being normally driven.
FIG. 3 is to explain the problem. FIG. 3 shows a most basic pixel configuration called a conductance control scheme which is preferably employed when EL elements are used as light-emitting elements. More specifically, the gate of a control transistor Tr1 constituted by an n-channel TFT is connected to a gate driver (not shown) through a scan selecting line A1, and the source is connected to a data driver (not shown) through a data line B1. The drain of the control transistor Tr1 is connected to the gate of a light-emitting drive transistor Tr2 constituted by a p-channel TFT, and one terminal of a charge storing capacitor Cs.
The source of the light-emitting drive transistor Tr2 is connected to the other terminal of the charge storing capacitor Cs and connected to a power supply line P1. An anode of an EL element E1 serving as a light-emitting element is connected to the drain of the light-emitting drive transistor, and the cathode of the EL element E1 is connected to a cathode-side power supply line. The sub-pixels having the above configuration constitute color pixels including the R, G, and B elements as combinations. The large number of color pixels in the form of a matrix in the horizontal and vertical directions on the display panel.
In the above pixel configuration, when an ON voltage is supplied to the gate of the control transistor Tr1 by a gate driver through the scan selecting line A1, the control transistor Tr1 causes a current corresponding to a data voltage from the data line B1 supplied to the source to flow from the source to the drain. Therefore, in the period in which the gate of the control transistor Tr1 has an ON voltage, the charge storing capacitor Cs is electrically charged, and the voltage is supplied to the gate of the light-emitting drive transistor Tr2.
therefore, the light-emitting drive transistor Tr2 is turned on based on a voltage between the gate and the source, and a drive voltage supplied through the power supply line P1, e.g., VHR is applied to the EL element E1 to luminescently drive the EL element.
On the other hand, when the gate of the control transistor Tr1 has an OFF voltage, the transistor is set in a cut-off state, and the drain of the control transistor Tr1 is set in an open state. The gate voltage of the light-emitting drive transistor Tr2 is held by electric charges accumulated in the charge storing capacitor Cs, a state in which the drive voltage VHR is applied to the EL element E1 is continued until the next scanning. In this manner, the emission of light from the EL element E1 is kept.
In the pixel configuration shown in FIG. 3, drive voltages (VHR, VHG, and VHB) having different values are applied through the power supply line P1 depending on the colors R, G, and B, respectively. As additionally described as an example in FIG. 3, reference symbol VHR denotes a drive voltage supplied to the sub-pixel for R. For example, the drive voltage is set at 7.0 V. Reference symbol VHG denotes a drive voltage supplied to the sub-pixel for G. For example, the drive voltage is set at 5.5 V. Reference symbol VHB denotes a drive voltage supplied to the sub-pixel for B. For example, the drive voltage is set at 6.0 V.
On the other hand, in the above configuration, voltages the levels of which are equal to those of the voltages VHR, VHG, and VHB are supplied to the sub-pixels for R, G, and B as a source supply voltage VHso supplied from the data driver to the source of the control transistor Tr1 through the data line B1. Therefore, when the control transistor Tr1 is turned on the configuration shown in FIG. 3, the device operates to turnoff the light-emitting drive transistor Tr2. In order to control the device to turn on the light-emitting drive transistor Tr2, as additionally described in FIG. 3, the device is designed to apply −2.0 V as a source supply voltage VLso.
In the above conditions, in order to set the control transistor Tr1 in a scan selecting state, the device must be designed such that a gate control voltage VHga (=9.0 V) having a value obtained by adding a threshold voltage of about 2.0 V at which the control transistor Tr1 can be turned on to the voltage VHR (7.0 V) which is the highest voltage in the voltages VHR, VHG, and VHB can be applied. On the other hand, in order to set the control transistor Tr1 in a non-scanning state, the device must be designed such that a gate control voltage VLga (=−4.0 V) lower than the source supply voltage VLso can be applied.
The light-emitting drive operation is continued based on the above voltage settings, forward voltages corresponding to R, G, and B gradually increase by aging. Accordingly, it is assumed that the voltages VHR, VHG, and VHB increase to 7.5 V, 6.0 V, and 8.0 V, respectively as additionally described as an example. In this case, when the gate control voltage VHga (=9.0 V) is set with respect to the maximum value (=8.0 v) of the source supply voltage VHso applied to the source of the control transistor Tr1, it is impossible to sufficiently turn on the control transistor Tr1. Therefore, an image is defectively displayed on the display panel.
In order to avoid the defect on the display panel, as a gate control voltage applied to the control transistor Tr1, a power supply voltage obtained by adding a threshold voltage at which the control transistor Tr1 can be turned on to the maximum accomplishment value of the voltages VHR, VHG, and VHB may be prepared from the beginning. However, continuous generation of the high voltage disadvantageously causes waste of a battery when use of a mobile device is supposed.